I'm assembling a receiver kit from QRP labs. It is an inexpensive kit designed to cover HF. This kit has a BS170 at the input which I assume is intended to disconnect the receiver when the transmitter is active. Here is the relevant part of the schematic:

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So I'm guessing the idea is when TX is high, the gate of Q1 is pulled low, thus isolating the RF input from the rest of the receiver (because the transmitter is active). When TX is low the transmitter is inactive, and Q1 is on, connecting the RF input to the band-pass filter and on to the rest of the receiver.

The transmitter normally paired with this has a nominal 200 mW output, and originally I was considering what modifications might be necessary to accommodate a higher power.

But then I got to thinking: even 200 mW corresponds to a peak voltage of 4.4V. That means when the transmitter is active, the drain of Q1 could be 4.4V below the source. This is more than sufficient to forward-bias the body diode.

I suppose this would only be a transient condition, since once the body diode begins to conduct the current will increase the voltage across C7 until the minimum voltage at the drain of Q1 is no longer sufficient to forward-bias the body diode. And at 200 mW, the negative consequences might be negligible.

Is my reasoning correct here? Does this seem like an OK design?

  • \$\begingroup\$ This is not an answer, but would it solve the QRP setting problem if the drain is pulled up to a +9 or something? But if that's the case, a larger power design would require a higher pull-up voltage and will not be very practical. That's probably why a PIN diode is used in this kind of RF switch. \$\endgroup\$
    – Ryuji
    Feb 1, 2022 at 21:59


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